In general, a fiber Bragg grating (FBG) sensor refers to a sensor using the reflection, refraction, diffraction, and transmission phenomena of light by transmitting light through an optical fiber. In such an FBG sensor, ultraviolet rays are selectively radiated to a glass optical fiber so that a pattern whose refractive index is periodically changed finely is formed in the length direction of the optical fiber.
Such an FBG sensor is characterized in that it has a low insertion loss because it can be formed as a filter within an optical fiber. Furthermore, the FBG sensor has a very low manufacturing cost in the case of mass production, and the bandwidth of the filter can be very small in size.
Other characteristics of the FBG sensor include that the FBG sensor is suitable for burial type measuring (durability) because it has a low possibility that it may be deteriorated due to corrosion and that an abnormal value is not generated due to moisture (reliability) because an optical signal is used. There is no influence, such as an electric field at a measuring place because an optical signal is transmitted to a measuring device. In particular, the FBG sensor is easy in a high voltage power environment (anti-noise), can be freely used around the inflammables because an electrical signal and electricity are not used (an anti-explosion property), can be easily used in long tunnels and large-scale structures because an optical signal has a very low transmission loss and can be measured in a range of several kilometers (applicability), can be used from an extremely low temperature (−270 degrees) to an extremely high temperature (several hundreds of degrees), and can be used in high temperature humidity and an extremely low temperature and high temperature (a temperature property). In particular, an FBG optical fiber sensor can measure stress within concrete like an existing electricity type measuring device and has very high economy because it can maintain a measuring function for more than 1 year even in a severe environment.
Such an FBG sensor includes a time division multiplexing (TDM) method using a time difference and a wave division multiplexing (WDM) method using a difference between waveforms. In the time division multiplexing method, a simple measuring system can be constructed because a maximum of 100 FBG sensors can be disposed in series in a single optical fiber. In contrast, the wave division multiplexing method is a method for identifying FBG in a higher layer of a reflection wavelength unique to FBG and can perform long-distance measuring.
Recently, the FBG sensor has been developed in a structural health monitoring (SHM) field, such as strain measuring, crack diagnosis, heat measuring, and pressure monitoring. The reason for this is that the FBG sensor has excellent advantages of an anti-electromagnetic property, a small size, corrosion resistant, and sensor multiplicity for a single fiber as described above. Furthermore, the FBG sensor may be used to measure parameters which significantly appear with respect to a change of a strain or temperature.
Conventional remote communication optical fibers include silica glass fibers coated in 2 to 3 polymer layers. The reason for this is that a coatless optical fiber is likely to be broken. A silica fiber coated with polymer provides high stiffness for a short period. If such fibers are subjected to stress in a wet environment, however, strength is reduced over a long time because a crack grows very slowly. In order to protect a silica glass fiber from the penetration of moisture, the coating of metal, such as aluminum, indium, tin, antimony, zinc, lead, copper, nickel, or gold, is performed on the optical fiber. Some of the fibers showed higher resistance against the penetration of moisture. Furthermore, some of the fibers showed higher stiffness compared to a polymer coating fiber. Some of the fibers can withstand a relatively higher temperature. Meanwhile, an optical fiber is coated in order to increase mechanical properties, such as Young's Modulus, the coefficient of thermal expansion, and a Poisson's ratio, to improve reliability, and to protect the optical fiber against a severe environment. Although such a technology for coating an optical fiber improves the characteristics of the optical fiber, but a technology for detecting a maximum strain of the object to be measured using the residual strain of a coated material has not been performed.